1. Field of the Invention
The present invention is related to a method for conversion of hydrocarbon compounds, and particularly to the use of a zeolite catalyst for olefin/paraffin alkylation or aromatic alkylation.
2. Background of the Art
Alkylation pertains to the chemical addition of an alkyl group to another molecule to form a larger molecule. Commercial alkylation processes can typically be aromatic alkylation or olefin/paraffin alkylation. Aromatic alkylation typically involves the production of alkylaromatic compounds (e.g., ethylbenzene, cumene, etc.) by alkylating an aromatic compound (e.g., benzene) with an olefin (e.g., ethylene, propylene, etc.). Olefin/paraffin alkylation pertains to the reaction between a saturated hydrocarbon with an olefin to produce a highly branched saturated hydrocarbon with a higher molecular weight, for example, alkylation of isobutane with 2-butene to produce a gasoline product having a high octane number.
Unlike the production of gasoline by cracking high molecular weight petroleum fractions such as gas oil or petroleum residua, alkylation gives a cleaner gasoline product without sulfur or nitrogen impurities. Moreover, alkylate gasoline has little or no aromatic content, which is a further environmental benefit.
Various alkylation processes are known and used throughout the petroleum industry. For example, alkylation is routinely carried out commercially by using liquid acid catalysts such as sulfuric acid or hydrofluoric acid. Alternatively, solid zeolite catalysts have been used. Use of zeolites avoids the disadvantages of using highly corrosive and toxic liquid acids. However, zeolites can suffer from deactivation caused by coking. Various processes for alkylating hydrocarbons using zeolite-containing catalysts are disclosed in U.S. Pat. Nos. 3,549,557 and 3,815,004. An alkylation process using an acid solid catalyst using zeolitic or non-zeolitic material is disclosed in U.S. Pat. No. 5,986,158, which is herein incorporated by reference.
Zeolites are porous crystalline materials characterized by submicroscopic channels of a particular size and/or configuration. Zeolites are typically composed of aluminosilicate, but zeolite materials have been made in a wide range of other compositions. The latter are commonly referred to as microporous materials. The channels, or pores are ordered and, as such, provide unique properties, which make zeolites useful as catalysts or absorbents in industrial processes. For example, zeolites can be used for filtering out smaller molecules, which become entrapped in the pores of the zeolite. Also, zeolites can function as shape selective catalysts that favor certain chemical conversions within the pores in accordance with the shape or size of the molecular reactants or products. Zeolites have also been useful for ion exchange, such as for water softening and selective recovery of heavy metals.
Synthetic zeolites are traditionally made from sources of silica and aluminum (silica and alumina “nutrients”) that react with each other, in the presence of materials that ensure highly alkaline conditions, such as water and OH−. Other zeolites can be borosilicates, ferrosilicates, and the like. Many of the crystallization steps are conducted in the presence of an inorganic directing agent, or an organic template, which induces a specific zeolite structure that cannot easily be formed in the absence of the directing agent or template. Many of the organic templates are quaternary ammonium salts, but can also be linear amines, alcohols, and a variety of other compounds. As a hydroxide, some directing agents introduce hydroxyl ions into the reaction system; however, the alkalinity is usually dictated by the amount of sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. The reaction typically involves a liquid gel phase in which rearrangements and transitions occur, such that a redistribution occurs between the alumina and silica nutrients, and molecular structures are formed which correspond to specific zeolites. Other metal oxides can also be included, such as titania-silica, boria-silica, etc. Some zeolites can only be made with organic templates. Other zeolites can only be made by means of an inorganic directing agent. Yet other zeolites can be made either by means of a hydrophilic (e.g., inorganic) directing agent or a hydrophobic (organic based) template.
Much of today's hydrocarbon processing technology is based on zeolite catalysts. Various zeolite catalysts are known in the art and possess well-arranged pore systems with uniform pore sizes. The term “medium pore” as applied to zeolites usually refers to zeolite structures having a pore size of 4-6 angstrom units (Å). “Large pore” zeolites include structures having a pore size of above 6 to about 12 Å. Since many hydrocarbon processing reactions at industrially relevant (i.e., high) conversion rates are limited by mass-transfer (specifically, intraparticle diffusion) , a catalyst particle with an ideal pore structure will facilitate transport of the reactants to active catalyst sites within the particle and transport of the products out of the catalyst particle, but still achieve the desired shape selective catalysis. Zeolite morphology, i.e., crystal size, is another parameter in diffusion-limited reactions.
Catalysts have a limited life, for example, because of coking. Catalyst deactivation usually requires a reactor shutdown and catalyst regeneration. Although the use of two reactors operated in a “swing mode” (i.e., alternating use of reactors wherein one is operated for alkylation while the other is down for regeneration) allows for continuous production, it is nevertheless preferable to perform alkylation reactions with zeolite catalysts which have a long life so as to minimize the economic losses incurred by reactor shut down and catalyst regeneration.